JP3.42 The possible winter impact from recurving tropical cyclones (2008

JP3.42

The possible winter impact from recurving tropical cyclones

Robert Hart, Florida State University, Tallahassee, FL; and L. F. Bosart and C. Hosler

The existence of wind on Earth largely results from the gradient of

temperature from the poles to the equator caused by varying insolation

with latitude. It has long been established and recently quantified,

that this redistribution of temperature is accomplished through oceanic

currents, atmospheric currents, and latent heat release. Implied within

all three is the role of tropical cyclones (TCs) that move out of the

tropics and poleward of the Hadley cell. It remains a curious climate

wildcard that although the pole to equator temperature gradient does not

directly produce TCs, TCs weaken this gradient when escaping the Hadley

cell. Yet, the magnitude of this TC role has remained nebulous and

qualitative.

By examining the winter midlatitude climate following varying recurving TC

seasons over 50 years, the role of recurving TCs in the redistribution of

energy is laterally approximated. The results suggest that a recurving

TC, on average, may weaken the subsequent role of winter baroclinic eddies

(and, implicitly, the mean pole-to-equator temperature gradient) by

approximately one percent. This reduction of meridional eddy temperature

flux implies a decrease in the available potential energy (APE) of the

atmosphere, either as a direct or indirect result of the anomalous

recurving TCs. Simultaneous with the decrease in APE, the static

stability (N) of the midlatitudes is decreased as a consequence of the

enhanced recurving TCs. These two competing changes result in a shift in

winter extratropical cyclone (XC) existence: Following an active

recurving TC season, intense XCs are slightly more frequent (owing to the

decrease in N), but weak to moderate XCs are far less frequent (owing to

the decrease in APE). It remains to be determined conclusively how the

accumulated impacts of the TCs, and/or the resulting large-scale

circulation changes, retain memory for months following the TC season.

Speculation is provided based on preliminary analyses of hemispheric

snowcover leading up to the start of the winter.

Improved seasonal forecasting of higher latitude winter climate anomalies

may be possible when the hemispheric frequency of recurving TCs is

anomalous. Unfortunately, with this frequency minimally predictable,

some component of winter midlatitude climate may be necessarily

unpredictable until the TC season has nearly ended. This study suggests a

view wherein TCs may be integral to the variability and forcing of

climate, rather than incidental and purely responsive to other forcings,

such as ENSO.